What is Herd Immunity?

Herd immunity (also called population immunity) is the indirect protection from an infectious disease that occurs when a sufficiently large proportion of a population becomes immune — either through vaccination or prior infection — thereby reducing the likelihood of transmission to those who are not immune. When enough people are immune, the pathogen cannot find sufficient susceptible hosts to sustain its spread.

The percentage of the population that must be immune to achieve herd immunity is called the herd immunity threshold (HIT). This threshold varies by disease and depends on the basic reproduction number (R0) — the average number of secondary infections produced by one infected person in a fully susceptible population. The formula is: HIT = 1 - (1/R0). For highly contagious diseases like measles (R0 = 12-18), the threshold is 92-95%, while for diseases with lower R0 values, a smaller proportion needs immunity.

The WHO supports achieving herd immunity through vaccination, not by allowing uncontrolled disease spread through a population. Uncontrolled spread causes unnecessary suffering, death, and potential healthcare system collapse, particularly among vulnerable groups — newborns, elderly, pregnant women, and immunocompromised individuals who cannot be vaccinated.

Herd immunity through vaccination is one of the greatest achievements of public health. It contributed to the eradication of smallpox (declared eradicated by WHO in 1980) and the near-elimination of polio. However, challenges such as vaccine hesitancy, pathogen mutation, and waning immunity complicate the achievement and maintenance of herd immunity, as demonstrated during the COVID-19 pandemic where evolving variants reduced vaccine effectiveness.

India's Universal Immunisation Programme (UIP), one of the largest in the world, and Mission Indradhanush (launched 2014) aim to achieve full immunisation coverage for preventable diseases including measles, polio, diphtheria, pertussis, and tetanus. The concept of ring vaccination — vaccinating all contacts of confirmed cases — was instrumental in the final phase of smallpox eradication and was also used during Ebola outbreaks in Africa.

Key Features

# Feature Details
1 Definition Indirect protection when a sufficient proportion of a population is immune
2 Achieved via Vaccination (preferred by WHO) or natural infection (not recommended)
3 Threshold formula HIT = 1 - (1/R0), where R0 = basic reproduction number
4 Measles threshold 92-95% (R0 = 12-18; one of the most contagious diseases)
5 Polio threshold ~80-85% (R0 = 5-7)
6 COVID-19 estimate 60-85% depending on variant; complicated by mutations and waning immunity
7 Protects vulnerable Shields newborns, elderly, and immunocompromised who cannot be vaccinated
8 R0 significance Higher R0 = higher percentage of population must be immune
9 Smallpox eradication Global vaccination achieved herd immunity; declared eradicated by WHO in 1980
10 Ring vaccination Targeted strategy — vaccinating contacts of infected persons to contain outbreaks
11 Vaccine hesitancy WHO listed it as a top 10 global health threat in 2019; undermines herd immunity
12 Waning immunity Over time, immunity may decline, requiring booster doses to maintain protection

UPSC Exam Corner

Prelims: Key Facts

  • Herd immunity threshold depends on R0 (basic reproduction number) of the disease
  • Measles requires 92-95% vaccination coverage for herd immunity (among the highest thresholds)
  • Smallpox was eradicated in 1980 — the only human disease eradicated through vaccination
  • WHO recommends achieving herd immunity through vaccination, not natural infection
  • Ring vaccination was a key strategy in smallpox eradication and Ebola containment
  • India's Universal Immunisation Programme (UIP) targets diseases including measles, polio, diphtheria, and tetanus
  • Vaccine hesitancy was listed by WHO as a top 10 global health threat (2019)
  • The formula HIT = 1 - (1/R0) means diseases with higher R0 need higher vaccination coverage
  • Mission Indradhanush (2014) aims to fully immunise children and pregnant women in India
  • Passive immunity (from mother or antibodies) provides temporary protection but does not contribute to herd immunity
  • Herd immunity does not protect against diseases like tetanus which are non-communicable

Mains: Probable Themes

  1. Explain the concept of herd immunity and its dependence on the basic reproduction number (R0)
  2. Discuss the role of herd immunity in India's vaccination programmes (UIP, Mission Indradhanush)
  3. Analyse the challenges to achieving herd immunity — vaccine hesitancy, pathogen mutations, and waning immunity
  4. How did the COVID-19 pandemic reshape our understanding of herd immunity?
  5. Evaluate the ethical dimensions of pursuing herd immunity through natural infection vs. vaccination

Important Connections

  • Public Health Policy: India's UIP and Mission Indradhanush aim to achieve herd immunity for preventable diseases
  • Ethics: Debate over natural infection vs. vaccination for herd immunity — WHO firmly supports vaccination route
  • Global Health Security: COVID-19 demonstrated how variants and waning immunity challenge herd immunity assumptions
  • International Cooperation: COVAX facility aimed to ensure equitable global vaccine distribution for achieving population immunity
  • Vaccine Diplomacy: India's "Vaccine Maitri" initiative supplied COVID-19 vaccines to countries worldwide, supporting global herd immunity efforts

Sources: WHO — Herd Immunity and COVID-19, Mayo Clinic — Herd Immunity, History of Vaccines — Herd Immunity